The present invention relates to a dual band reflector antenna and in particular to a dual band feed having a mode transducer coupled to a feed waveguide and integral to a corrugated horn for providing near ideal performance at both frequency bands.
The performance of a communications terminal is related to the gain of the antenna, the noise figure of the receiver, and the output power of the transmitter. By increasing the gain of the antenna, the performance, and therefore cost of the receiver and transmitter can be reduced while maintaining the same system performance. Since the size of the antenna is typically limited by volume or pedestal constraints, the only means of increasing the antenna gain is to improve the antenna efficiency. To optimize the antenna efficiency, a feed for a reflector system must produce rationally symmetric radiation patterns and have coincident E and H plane phase centers. In an optimal dual band reflector antenna, a single feed must obtain these requirements while maintaining radiation characteristics at both frequency bands.
In the prior art U.S. Pat. No. 3,922,621 by R. W. Gruner, issued Nov. 25, 1975, teaches a 6-port directional orthogonal mode transducer comprising an inner circular waveguide for propagating transmit signals and an outer, circular, coaxial waveguide for propagating lower frequency receive signals. The terminal end of the outer waveguide is joined to an enlarged, cylindrical coupling section provided with a plurality of spaced, inwardly projecting corrugations in the form of washer-like annular rings. The corrugations, when properly dimensioned, establish surface reactance conditions, that result in an inner circular field distribution at the transmit frequency and a surrounding annular field distribution at the receive frequency. Although the transducer provides isolation between the transmit and receive channels, it does not realize the mode structures needed for optimal feedhorn performance.
In the prior art other dual band feeds typically employ separate radiating structures, or configurations, for each frequency band. A typical approach is to utilize a corrugated or multi-mode horn and a dielectric polyrod for the low and high bands, respectively. Such a configuration achieves the desired performance at the low band, but not at the high band. In these feeds the dielectric polyrod does not function as a transition into the corrugated or multimode horn, but rather as a radiator for high band. The dielectric polyrod is narrow band and does not produce rotationally symmetric patterns or stable coincident phase centers.
Another approach is to utilize the same horn operating single mode and multi-mode for the low and high bands, respectively. The multi-mode operation produces non-ideal, but acceptable, performance at the high band, but the low band is far from ideal. Current dual band feeds achieve the desired radiation performance at one band by compromising performance at the other band.